"The New Hacker's Dictionary" defines (poorly, as usual) a "troll" like this:

troll v.,n. To utter a posting on Usenet designed to attract stupid responses or flames. May derive from the phrase "trolling for newbies" or some similar construction. The well-constructed troll is a post that induces lots of newbies and flamers to make themselves look even more like idiots than they already do, while subtly conveying to the more savvy and experienced that it is in fact a deliberate troll. If you don't fall for the joke, you get to be in on it.

Some people claim that the troll is properly a narrower category than flame bait, that a troll is categorized by containing some assertion that is wrong but not overtly controversial.

1. troll \'tro-l\ v.t. [ME trollen] 5a: to angle for with a hook and line drawn through the water 5b: to angle in {~ lakes} 5c: to pull through the water in angling {~ a lure} v.i. 2: to fish esp. by trolling a hook

2. troll n 1: a lure or a line with its lure and hook used in trolling

Ah, "The New Hacker's Dictionary". Never explain clearly when you can explain obscurely and look K00LER than your audience. Not merely a complete ripoff of, but an embarrassing ripoff of the original Jargon File.